TheSkySearchers

So, considering my previously posted issues with the front end of my Nexstar 8i SE, I thought it prudent to take some measurements. When reinstalling the corrector plate, I had assumed that the goal was to install spacers so that the outside of the glass was equidistant from the inside of the front casting.

That turns out to have been a poor assumption. When i actually measured the glass, I discovered that the hole for the secondary was nearly 50 thousandths (of an inch) off center. The good news is that the glass is smaller than the front casting by a margin that allows one to center the secondary on the tube with the use of different sized spacers. The secondary is a snug fit in the hole in the glass, so no movement there.

The best I could do was to set the glass in place, and move it until the plastic housing was equidistant from the rim. Then I put spacers in to hold it that way. I put the hold down ring in snug, measured my spacing again. then tightened it down.

What I achieved is that the plastic housing for the secondary is centered on the tube. Whether the secondary mirror is in the center of the plastic housing is another question entirely.

I am still assuming that the primary mirror and the rest of the back end are centered on the tube, as the eyepiece opening appears to be milled on center. Of course, it may be that the hole for the secondary was bored off center to match a measured offset or irregularity in the primary.

Anyway, I found the fact that the manufacturer did not make much of an effort in precision interesting. Right now, I just want to get everything aligned as best I can. If I have time in the future, I might add some adjustments to center and hold the corrector plate with precision, and perhaps manufacture a better housing for the secondary mirror.

It sounds like you are going the right direction.

I did find a few topics on installing and collimating the 8i se. Hope these help!

https://www.nexstarsite.com/OddsNEnds/S ... emoval.htm

viewtopic.php?p=6156#p6156

Thanks, Gordon. I noticed a line in the first linked article-
"The next thing I noticed is that Celestron was no longer using spacers to center the corrector in the cell. In fact, it is apparent it was simply placed in the cell and where ever it landed, so be it."
Which tells me that he probably made the same assumption I started with, that the corrector plate is a precision made, symmetrical object.

I think I am going to try something different. As it is manufactured, the front casting is held on by a series of screws which have nuts on the inside of the tube. This means that you need to put the casting on before the corrector plate, just to hold the nuts. If I used captive nuts in the tube, I could center the secondary with real precision, then mount the casting back on. I bet if the secondary mirror was centered to within .0001", I would not have further issues. I could probably even adjust the secondary to level in the mount, and indicate it to ensure that the apex of the mirror is dead center in the casting, then install it and be close to collimated.

MaxBlancke wrote: ↑Tue Feb 07, 2023 10:37 pm Which tells me that he probably made the same assumption I started with, that the corrector plate is a precision made, symmetrical object.

But it isn't!
I own a C11 and know for a fact that the CP (corrector plate) is precision placed based on the results from actual bench testing of each OTA. I sent in my C11 for a "tune-up" after I bought it used. I was told they (the technicians) bench-test the placement of the CP. I also received a paper report showing the results of that bench test.

Just because there are no marks doesn't mean the CP was not precisely placed. It means, they (Celestron) want you to send in your OTA (for whatever reason) to keep their repair center in the black!

JayTee wrote: ↑Wed Feb 08, 2023 1:06 am

MaxBlancke wrote: ↑Tue Feb 07, 2023 10:37 pm Which tells me that he probably made the same assumption I started with, that the corrector plate is a precision made, symmetrical object.

But it isn't!
I own a C11 and know for a fact that the CP (corrector plate) is precision placed based on the results from actual bench testing of each OTA. I sent in my C11 for a "tune-up" after I bought it used. I was told they (the technicians) bench-test the placement of the CP. I also received a paper report showing the results of that bench test.

Just because there are no marks doesn't mean the CP was not precisely placed.

I do not claim that it was not precisely placed, but that it was imprecisely manufactured. The interior of the front casting is reasonably round, within the sorts of tolerances one might expect to see in a good casting. The flat upon which the corrector plate sits seems to be machined, and perpendicular to the axis of the tube. The corrector plate is more or less round. The hole for the secondary mirror is quite round and a good fit for the secondary.
However, the hole for the secondary is not in the center of the plate. It is off by nearly 1/16" or 1.27mm. In my world, 50 thousandths of an inch is a large error indeed.

If they manufactured the component parts to tighter tolerances, assembly (or reassembly) would be a straightforward affair, and not require artistry. I see the same sort of thing with high-end antique automobiles. They are wonderfully fitted, with equal seams and gaps between the doors and the body. However, if you try to replace a body panel with one from another car of the same make and model, the panel will not be likely to fit. Back at the factory, they probably had to sort through a whole pile of parts to get a decent match, and it still took an artist to fit it properly.

It may well be that the hole in the CP was cut off-center to match an alignment error in the primary or primary tube. But that just leads me to ask why those components would be so far off in the first place. Anyway, if the center whole were centered, the technician would still be able to use spacers to offset the corrector plate as needed.

I don't really mean to complain, I enjoy such work. It just surprises me.

MaxBlancke wrote: ↑Wed Feb 08, 2023 2:22 am However, the hole for the secondary is not in the center of the plate. It is off by nearly 1/16" or 1.27mm. In my world, 50 thousandths of an inch is a large error indeed.

If they manufactured the component parts to tighter tolerances, assembly (or reassembly) would be a straightforward affair, and not require artistry.

But it does because it's amateur-grade optics.

I'm not trying to dispute you, I'm just trying to show that many parts of the OTA are imprecise in a relatively inexpensive commercial product.

Have you measured the precision of the focuser that moves the entire primary mirror? I'll save you some time, it's scary!

So if the primary mirror can't even be moved precisely, then artistry is required.

Lastly, my C11 primary mirror moves around even more because it weighs quite a bit than your 8" mirror. And my C11 is an expensive optical device.

Just some thoughts on my part.

Cheers,

JayTee wrote: ↑Wed Feb 08, 2023 2:56 am Have you measured the precision of the focuser

I have not got that far yet. I did measure the primary tube today. It does not run parallel to the housing tube. So that puts me in the situation where I have to center the secondary on the primary tube, instead of centering it in the middle of the front casting.
What I did, was remove the secondary mirror assembly from it's plastic housing. It is actually a well made piece. Anyway, I leveled the mirror to it's base, then located the center of the mirror. I put a little mark (which will clean off later) right in the center of the mirror. Then I assembled the front end without spacers around the corrector plate.
Then, I put a borescope in the viewing tube, and was able to move the corrector plate so that the viewing tube lines up exactly with the center of the secondary mirror.

Tomorrow, I plan to measure and mark the corrector plate position, take it back off, clean the mirror, and reassemble everything. That should put me back in business. I don't have any cork, so I plan on using rubber spacers.

Sounds like you are doing fantastic work on your OTA. Keep us updated, especially when you take it out for its first light -- again!

Cheers,

Thanks for the help and encouragement. I got it back together today, and have played with it in daylight.

The solution for me turned out to be marking the center of the secondary, and lining everything up by using a borescope in the viewing tube. Once it was right, I took good measurements of the best position and orientation of the corrector plate. Then I disassembled the front end, cleaned everything really well, and put it all together again.
I dispensed with the cork spacers, and opted instead to put six little blobs of 3M 5200 sealer to hold the plate in position. I also used a film of 5200 on the gaskets for the secondary, so it is unlikely to loosen itself up again.

I am optimistic that things are good, for the time being.

MaxBlancke wrote: ↑Sat Feb 11, 2023 1:31 am The solution for me turned out to be marking the center of the secondary and lining everything up by using a borescope in the viewing tube. Once it was right, I took good measurements of the best position and orientation of the corrector plate. Then I disassembled the front end, cleaned everything really well, and put it all together again.
I dispensed with the cork spacers and opted instead to put six little blobs of 3M 5200 sealer to hold the plate in position. I also used a film of 5200 on the gaskets for the secondary, so it is unlikely to loosen itself up again.

Max, let me say you have some extraordinary technical capabilities! The vast majority of us cannot do what you just described and accomplished.

Cheers to you,